This invention relates to a motion compensation prediction picture producing device for use in a motion picture coding device which carries out high-efficiency coding of a digital motion picture signal representative of successive pictures, each of which may be either a frame or a field. The motion compensation prediction picture producing device is for predictively producing a motion compensation prediction picture predictive of a current picture by using a previous picture which precedes the current picture among the successive pictures.
As a high-efficiency coding technique, a motion compensation technique is known in the art. In the motion compensation technique, each of successive pictures is divided into a predetermined number of blocks of pixels (picture elements). A motion vector is detected which represents a movement of each block between two successive pictures. In this event, the motion vector is detected between an input picture (namely, a current picture) and a reference picture (namely, a previous picture) for each block of the input picture. In accordance with the motion vector and a position of each block of the input picture, a reference block is extracted from the reference picture for each block of the input picture. A prediction picture which is predictive of the input picture is produced by using the reference block extracted for each block of the input picture. The prediction picture is generally produced by using each reference block as it is.
In Japanese Unexamined Patent Publication No. 4-245884(namely, 245884/1992), a predictive coding system is disclosed which uses an overlapped motion compensation technique. In such an overlapped motion compensation technique, the prediction picture is produced by the use of a processed reference block into which each reference block extracted for each block of the input picture is processed by the use of a window function in the manner which will later be described. The overlapped motion compensation achieve a smoothing effect of smoothing discontinuities at block boundaries of the reference blocks and a prediction error suppressing effect of suppressing a prediction error as compared with a conventional motion compensation technique using no window function. Furthermore, optimization of the window function for the overlapped motion compensation is described in an article entitled "Overlapped Block Motion Compensation" written by Cheung Auyeung et al and contributed to Proc. of Visual Communications and Image Processing '92, SPIE Vol. 1818, Nov. 1992, pages 561-572 and in another article entitled "Overlapped Motion Compensation for Low Bit-Rate Video Coding" written by Jiro KATTO et al and contributed to Int. Workshop on Multimedia Mobile Communications, Dec. 1993, pages B.1.3-1-B.1.3-6.
In any one of the prior arts, use may be made of the window function which is determined independently of a characteristic or feature of the input or current picture. Inasmuch as the window function does not explicitly reflect the characteristic of the input picture, improvement in prediction efficiency is not satisfactory. In addition, a method of adaptively changing of the window function in dependence upon the characteristic of the input picture is not established. As a consequence, it is impossible to provide the window function for the overlapped motion compensation in dependence upon the characteristic of the input picture so as to reduce the prediction error. Thus, any one of the prior arts never teaches the use of the window function which varies in response to the input or the current picture.